Apart from carbon monoxide (CO) and hydrocarbons (HC), nitric oxides (NOx) are among the environmentally harmful, directly emitted, primary injurious substances which are generated during the operation of internal combustion engines, in particular diesel engines. The use of three-way catalysts, such as are used in Otto engines and gas engines, cannot be used in the exhaust of diesel engines due to oxygen excess. For this reason, for the reduction of the nitric oxide emission in diesel engines a selectively operating SCR catalyst (Selective Catalytic Reduction Catalyst) has been developed in which, in the presence of an added reducing agent, namely ammonia (NH3), the expelled nitric oxides are reduced to N2 and H2O.
The ammonia required for carrying out the reduction of the nitric oxides can be transported along on board of the motor vehicle in different forms. Pure ammonia in gaseous or in liquid phase can be carried. To avoid problems in handling, the pure ammonia is generally stored in a tank on the motor vehicle in the bound form. The ammonia carried in bound form is hydrolytically split to release the bound ammonia either in the exhaust gas system or previously.
One such reducing agent employed is an aqueous solution of urea. The urea solution is stored in a reducing agent tank and is connected with the exhaust gas system of the internal combustion engine via a supply line. A dosing valve disposed on the exhaust gas system serves to deliver the required quantity of urea that is immediately gasified due to the temperatures obtained in the exhaust gas system. The gasification releases the ammonia required for carrying out the nitric oxide reduction.
Due to the disposition of the dosing valve immediately on the exhaust gas system, the input side of the dosing valve must be cooled to prevent the liquid urea solution decomposing or crystallizing out due to the high temperatures. Such reducing agent dosing devices are known in the art, for example from DE 198 56 366 C1.
According to a further known implementation the dosing valve is disposed directly beneath the reducing agent tank and is connected with the exhaust gas system over a delivery line that is several meters long. Compressed air from the air compressor for the brake system transports the quantity of urea solution in the delivery line. The dosed urea solution quantity is, therefore, transported in the supply line as an aerosol. However, the problem with this implementation is that the several meter long supply line, which carries the aerosolized reducing agent, can have a relatively small diameter due to the quantity of air which can be branched off from the air compressors. Therefore, in this implementation a fundamental hazard is reducing agent precipitating out of the aerosol onto the walls of the supply line, crystallizing on the walls and impairing or eliminating the through flow capability.
There are disadvantages of using a reducing agent dosing device which delivers liquid reducing agent into the exhaust gas system and which has the dosing valve positioned directly on the exhaust gas system, as described in DE 198 56 366 C1. One disadvantage is that the termination opening of the dosing valve must be oriented so that the reducing agent jet output dose output is parallel to the direction of flow of the exhaust gas. This prevents the delivered jet of reducing agent contacting one of the encompassing walls of the exhaust gas system and crystallizing on these sites. This would reduce the flow cross-section of the exhaust gas pipe and may even cause an obstruction. For this reason it is necessary to set a dosing valve, such as described in DE 198 56 366 C1, into an exhaust gas system elbow. This is not always possible. In particular it is not desirable to introduce a corresponding curvature into the exhaust gas system solely for the positioning of the dosing valve.
Building on this discussed prior art, the present invention addresses the problem of developing an above described reducing agent dosing device such that effective reducing agent dosing into the exhaust gas system of an internal combustion engine is possible without having to tolerate the previously demonstrated disadvantages in prior art.
This problem is solved according to the present invention by having the dosing valve spaced from the exhaust gas system in the supply line connecting the reducing agent tank with the exhaust gas system. The output of the dosing valve ends in a mixing chamber through which, during operation of the internal combustion engine, flows compressed air in the direction to the exhaust gas system, mixing the reducing agent delivered by the dosing valve with an air stream. The compressed air is provided by the charge air of a charging group of the internal combustion engine